changes all url string to URI type

Fixes testmycode#77

refactor strings to URI changes

arrange two more imports

fix indentation

fix indentation and import orders

fixed little bug

Brought back zip_url because it got lost after rebasing

src/main/java/fi/helsinki/cs/tmc/core/domain/Exercise.java

@@ -29,12 +29,12 @@ public class Exercise implements Serializable {
 
     private String checksum; //: "406f2f0690550c6dea94f319b2b1580c",
 
+    @SerializedName("zip_url")
+    private URI zipUrl;
+
     @SerializedName("return_url")
     private URI returnUrl; //: "https://tmc.mooc.fi/staging/exercises/284/submissions.json",
 
-    @SerializedName("zip_url")
-    private URI zipUrl; //": "https://tmc.mooc.fi/staging/exercises/284.zip",
-
     /**
      * The URL the solution can be downloaded from (admins only).
      */

src/test/java/fi/helsinki/cs/tmc/core/communication/SubmissionPollerTest.java

@@ -20,6 +20,9 @@
 import org.junit.Before;
 import org.junit.Test;
 import org.mockito.Mockito;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
 
 public class SubmissionPollerTest {